When I was a shift supervisor on the over the horizon radar on Amchitka
Island we used to see a brief window around dawn when DX propagation
improved. This window corresponded roughly to that time when the rising sun
was already "shining" on the F layer, but was not yet high enough to
illuminate the D and E layers. There was an improvement in high angle
signals in the western direction, that is into the darkness. For many high
angle hops, the path would have to be almost all over water, too. This
effect did not occur at the expense of any low angle signals, if they were
there. We did not measure radiation angle, we measured hop distance which
is a function of takeoff angle. In this context, low angle would be
somthing like 1200 miles or more per hop, high angle would be less than 1200
miles.
A simple way to look at this might be: For a given frequency, certain
angles of radiation will yield paths to certain areas. If the ionosphere
changes to allow a greater range of angles, then paths will open to more areas.
My observations were done with huge vertical arrays, so that says nothing
about the polarization question. Additionally, there are many other factors
which could affect antenna performance. Efficiency, far field ground
conductivity, geography, geomagnetic location, noise, etc. Nonetheless, I
would think that at least some of high angle reports around dawn are, in
fact, real.
Dan KL7Y
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